1. Field of the Invention
The present invention relates to a potentiometer provided with giant magnetoresistive effect elements that demonstrate very large resistance variations in response to the variation of external magnetic fields.
2. Related Art
A magnetic field sensor using giant magnetoresistive effect elements is disclosed in, for example, the Japanese Published Unexamined Patent Application No. Hei 8-226960, in which four giant magnetoresistive effect elements are electrically connected in a bridge circuit.
As shown in FIG. 17, a magnetic field sensor A disclosed in this application comprises separately located giant magnetoresistive effect elements 1, 2, 3, 4, in which the giant magnetoresistive effect elements 1, 2 are connected by a lead 5, the giant magnetoresistive effect elements 1, 3 are connected by a lead 6, the giant magnetoresistive effect elements 3, 4 are connected by a lead 7, the giant magnetoresistive effect elements 2, 4 are connected by a lead 8, an input terminal 10 is connected to the lead 6, an input terminal 11 is connected to the lead 8, an output terminal 12 is connected to the lead 5, and an output terminal 13 is connected to the lead 7.
And, the giant magnetoresistive effect elements 1, 2, 3, 4 each assume a sandwich structure in which a non-magnetic layer 15 is interpolated between an upper and a lower ferromagnetic layers 16, 17, and an anti-ferromagnetic exchange bias layer 18 is formed on the ferromagnetic layer (pinned magnetic layer) 16, whereby the exchange coupling generated by this exchange bias layer 18 pins the magnetization axis of the ferromagnetic layer 16 in one direction. Further, the orientation of magnetization axis of the ferromagnetic layer (free magnetic layer) 17 on the other side is made to freely rotate in accordance with the orientation of the external magnetic field; for example, it is made to freely rotate on the horizontal plane including the ferromagnetic layer 17.
Further, in the magnetic field sensor A having the structure shown in FIG. 17, the orientation of magnetization axis of the pinned magnetic layer 16 of the giant magnetoresistive effect element 1 faces forward as shown by the arrow 20 in FIG. 17, the orientation of magnetization axis of the pinned ferromagnetic layer 16 of the giant magnetoresistive effect element 2 faces backward as shown by the arrow 21, the orientation of magnetization axis of the pinned magnetic layer 16 of the giant magnetoresistive effect element 3 faces backward as shown by the arrow 23, and the orientation of magnetization axis of the pinned magnetic layer 16 of the giant magnetoresistive effect element 4 faces forward. And, the orientation of magnetization axis of the free magnetic layer 17 of each of the giant magnetoresistive effect elements 1, 2, 3, 4 faces to the right as shown by the arrow 24 in FIG. 17, in the state that the external magnetic field is not exerted.
In the magnetic field sensor A shown in FIG. 17, when an external magnetic field H is exerted, in the first and fourth giant magnetoresistive effect elements 1, 4, for example, the magnetization axis 24 of the free magnetic layer 17 rotates by a specific angle d as shown in FIG. 18, in accordance with the external magnetic field H; accordingly, the relation of angle to the magnetization axis 20 of the pinned magnetic layer 16 varies to effect a resistance variation. And, since the orientations of magnetization axes of the pinned magnetic layers 16 of the first and fourth giant magnetoresistive effect elements 1, 4 face opposite with the difference of 180.degree. to the orientations of magnetization axes of the pinned magnetic layers 16 of the second and third giant magnetoresistive effect elements 2, 3, the resistance variation involving a phase difference can be acquired.
In the magnetic field sensor A electrically connected in a bridge circuit shown in FIG. 17, the orientations of magnetization axes are specified as shown by each of the arrows, since the differential output has to be obtained from the giant magnetoresistive effect elements 1, 2, 3, 4 when the magnetization orientations of the free magnetic layers 17 vary in response to the external magnetic field H, and in the giant magnetoresistive effect elements 1, 2, 3, 4 located right and left, upper and lower in FIG. 17, the magnetization axes are needed to be pinned in antiparallel directions such that any of two adjacent elements are magnetized in the opposite directions with 180.degree..
In order to achieve the structure shown in FIG. 17, it is imperative to form the giant magnetoresistive effect elements 1, 2, 3, 4 adjacently on a substrate, and fix the orientations of magnetization axes of the pinned magnetic layers 16 of any adjacent two of giant magnetoresistive effect elements opposite each other with the difference of 180.degree..
Further, in order to control the magnetization orientations of the pinned magnetic layers 16 of this type, and adjust the lattice magnetization of the exchange bias layer 18, it is imperative to apply a magnetic field of a specific direction to the exchange bias layer 18 while it is heated at a higher temperature than the so-called blocking temperature at which the ferromagnetism disappears, and in addition to conduct a heat treatment to cool while this magnetic field is maintained under application.
However, in the structure shown in FIG. 17, since the magnetization orientations of the exchange bias layers 18 must be shifted by 180.degree. to one another for any two of the giant magnetoresistive effect elements 1, 2, 3, 4, the directions of the magnetic fields must be controlled individually for each of the giant magnetoresistive effect elements adjacently formed on a substrate. Since the method of applying a magnetic field simply from outside by using the magnetic field generator such as an electromagnet or the like allows application of the magnetic field only in one direction, it is very difficult to fabricate the structure shown in FIG. 17, which is a problem.
The technique disclosed in the Japanese Published Unexamined Patent Application No. Hei 8-226960 indicates that the structure shown in FIG. 17 can be achieved by depositing conductive layers individually along each of the giant magnetoresistive effect elements 1, 2, 3, 4 adjacently formed on a substrate, flowing currents in each of these conductive layers in different directions to individually generate magnetic fields of different directions from each of the conductive layers, and conducting the foregoing heat treatment. However, even if it is desired to generate high magnetic fields by applying high currents to the conductive films in order to control the lattice magnetization of the exchange bias layers 18, it is difficult to flow high currents through the thin conductive films that are deposited with the giant magnetoresistive effect elements on the substrate, and difficult to generate the magnetic fields from the conductive films, which are sufficient for the subsequent processes. Further, since the magnetic fields are exerted on the giant magnetoresistive effect elements 1, 2, 3, 4 adjacently formed on a substrate, in each different directions from a plurality of the conductive films, it is extremely difficult to individually apply the high magnetic fields to each of the exchange bias layers 18 of the giant magnetoresistive effect elements 1, 2, 3, 4.
As mentioned above, the magnetic field sensor A shown in FIG. 17 possesses an excellent function as a magnetic sensor; however in reality, to form the films on a substrate and fabricate the magnetic field sensor A involves extremely delicate processes to apply the magnetic fields and heat processes, making the fabrication difficult, and the structure causes a problem for wider applications.
Further, as to the applications of the magnetic field sensor A shown in FIG. 17, the Japanese Published Unexamined Patent Application No. Hei 8-226960 only suggests the applications to linear and rotary encoders, proximity sensors, geomagnetic magnetometers, and the like. And, there are not any concrete suggestions as to which equipment and fields the structure of the magnetic field sensor A is to be applied to.
On the other hand, as an example of products applying the magnetism, a magnetic potentiometer using the Hall elements is well known. The potentiometer of this type uses the Hall elements responsive to the variations of the magnetic field as the detecting elements, however the signal output obtained from the Hall elements of this type is extremely low; accordingly, a new structure has been expected which can produce a higher signal output.